(1) Field of the Invention
This invention relates to porous membranes useful, for example, in the filtration of fluids to remove any fine particles present therein.
(2) Description of the Prior Art
Various types of porous membranes are known in the prior art. They include (1) asymmetrical membranes which are made by the wet coagulation process and consist of a dense layer and a void layer; (2) homogeneous membranes which are made by forming a molten crystalline polyolefin and then cold-stretching the resulting preform, and include three-dimensionally interconnecting interstices surrounded by fibrils and knotted portions; and (3) porous membranes of the straight pore type which are made by bombarding a polymer film of, for example, polycarbonate with charged particles, and include cylindrical pores extending therethrough in the direction of the membrane thickness.
In the aforesaid asymmetrical membranes, however, the pores present in the dense layer generally have a wide pore diameter distribution and a very tortuous and complicated form. Moreover, these asymmetrical membranes are usually made by a wet process. For these reasons, these asymmetrical membranes have the disadvantage that, once dried, they undergo a structural change and hence exhibit a marked increase in filtering resistance. In order to avoid this, a complicated procedure is required which comprises treating the membranes with a special fluid such as glycerol and then drying them carefully. They are further disadvantageous in that their filtering resistance is high, the material to be filtered is easily retained within the pores of the dense layer, difficulties are encountered in removing the material retained with the pores to restore their filtering function, and hence their service life is short.
The aforesaid porous membranes having three-dimensionally interconnecting pores are characterized by excellent resistance to clogging. This is because, even if some pores become clogged, filtration can take place by other routes. However, they yet remain to be improved in some respects, because their fractionation accuracy is not satisfactorily high because of their volume filtration nature and because it is difficult to restore their filtering function.
The aforesaid membranes of the straight-pore type have the disadvantage that their surface pore density is limited to a maximum of 20% and, therefore, it is impossible to enhance their fluid permeability per unit membrane area. At present, porous membranes of the straight-pore type having a high surface pore density are not available.